1. Field of the Invention
This invention relates broadly to orthopedic plates. More particularly, this invention relates to plates that locks a fastener in a desired angular orientation relative to plate to maintain the desired angular orientation.
2. State of the Art
There has been a high level of interest by the orthopedic industry to develop locking fasteners for bone plates. The exact requirements vary for the locking fasteners used with each type of bone plate and the surgical indication. However, orthopedic surgeons generally prefer locking fasteners that are easy to insert and remove, reliably lock to the plate and are atraumatic to surrounding soft tissues. In addition, it is advantageous in certain situations to have a fastener that can be set in a surgeon-defined angle relative to the plate.
One such system meeting all the aforementioned requirements is the POLYAX locking screw system available from DePuy Spine, Inc., which is described in U.S. Pat. No. 5,954,722 to Bono et al. The POLYAX locking screw system allows the surgeon to angle the axis of the bone screw up to 15 degrees in any direction relative to the hole axis, and then to lock the screw head to the plate to maintain that angle. The POLYAX system includes a split bushing having an outer spherical shape that is assembled into a plate hole having a matching spherical contour. The bushing has a conical (6° included angle) threaded hole that receives a matching threaded head of the bone screw. A small pin pressed into a hole located on the perimeter of the plate hole aligns with the split of the bushing to prevent rotation of the bushing as the bone screw head engages the conical threaded hole of the bushing. Tightening of the bone screw expands the bushing against the inside surface of the plate hole and locks the screw to the plate. Both the POLYAX bushing and the bone plate are formed from a titanium alloy (Ti-6Al-4V) of similar hardness and smooth features.
The POLYAX locking plates can be used in various applications. By way of example, vertebral plates and periarticular plates can be assembled as POLYAX systems. With respect to a periarticular plate, bushings are currently provided in plates having a thickness of about 3 mm.
Proposed bone plates such as for elbow fractures have portions with a substantially smaller thickness of about 2 mm, one-third smaller in thickness. Portions of the bushing and screw head of the current POLYAX system, if assembled into a 2 mm thick bone plate and then angled to the maximum extent, would undesirably extend beyond the top and bottom surfaces of the bone plate. This may cause irritation of surrounding soft tissues or prevent the plate from seating properly against the bone. Simply reducing the overall height of the bushing would help to make the portions of the bushing and screw head less “proud” to the plate surfaces. However this also would reduce the area of the interfacing surfaces of the bushing and the plate hole, thereby reducing the frictional engagement and the overall locking force.